<div class="parent">
   <div class="panel-heading">父组件</div>
   <div class="panel-body">
     <app-child-two>
       <h3>这是父层投影进来的内容</h3>
       <app-child-three></app-child-three>
       <p>这是底部内容</p>
     </app-child-two>
   </div>
</div>
